The invention generally relates to adaptive reservation of channelization codes and/or power for downlink, preferably for the DSCH (downlink shared channel) and/or the HS-DSCH (high speed downlink shared channel) which is part of the HSDPA (High Speed Downlink Packet Access) concept.
The downlink shared channel (DSCH) in UTRAN (Universal Terrestrial PAN (Radio Access Network)) is a packet channel which is time shared by multiple users. The DSCH may be mapped to one or multiple PDSCHs (Physical Downlink Shared Channels) having a spreading factor between 4 and 256. The DSCH offers high data-rates and fast scheduling with bit rate modification every 10 ms, which makes it attractive for bursty packet applications such as web browsing, etc. The HS-DSCH can be regarded as an enhanced DSCH which offers bit rate modification every 2 ms as well as adaptive modulation and coding. The HS-DSCH is mapped to HS-PDSCH (high speed physical downlink shared channel).
In order to facilitate fast bit rate modification, a certain set of channelization codes is usually reserved for each DSCH as illustrated in FIG. 1. This means that time delays due to release and setup of new codes can be avoided. However, this is done at the expense of potentially wasting part of the limited code resources when the PDSCH is using the higher spreading factors. It was therefore of advantage when the reserved codes are adjusted adaptively according to traffic load in the cell, among others.
Link Adaptation (LA) techniques are commonly used for control (i.e. bit rate selection) of the DSCH. LA aims at minimizing the transmit power variations of the PDSCHs by transmitting with lower bit rates to UEs (User Equipments) which are far from the BS (Base Station) compared to those close to the BS. The selected bit rate for each UE can be expressed as a function of the power allowed for the PDSCH and the associated DPCH (PtxPDSCHallowed & PtxDPCH), the planned EbNo's for the channels (ρPDSCH & ρDPCH) , and the bit rate of the associated DPCH (RDCH) (DPCH=Dedicated Physical Channel). According to the LA criteria, the bit rate to be allocated a user is therefore expressed as
                              R                      DSCH            ,            LA                          =                  Round          ⁢                      {                                                                                P                                          tx                      ⁢                                                                                          ⁢                      PDSCHallowed                                                        ⁢                                      ρ                    DPCH                                                                                        P                    txDPCH                                    ⁢                                      ρ                    PDSCH                                                              ⁢                              R                DCH                                      }                                              (        1        )            where Round{ } denotes truncation to the nearest possible bit rate. That can e.g. be 32 kbps, 64 kbps, etc., depending on the reserved channelization codes. Knowledge of PtxDPCH is obtained through average measurements.
Provided that there are sufficient data to be transmitted on the PBSCH, the LA algorithm will automatically result in the following property:E{PtxPDSCH}≅PtxPDSCHallowed,  (2)where the mathematical operator E{ } takes the expectation over time. If the relation in equation (2) is false, then it indicates that the DSCH is being poorly exploited. Various reasons for this could be that too much power has been reserved for the PDSCH under the given traffic load or that channelization code blocking is occurring, where the intended LA bit rates according to equation (1) are limited by the minimum allowed spreading factor, i.e. RDSCH<RDSCH,LA. Even though equation (2) is valid, there might still be room for optimization by allowing a larger fraction of power to be reserved for the PDSCH. Effective utilization of the DSCH by using LA does therefore depend on the settings of PtxDSCHallowed and the spreading factor of the root channelization code, SFmin.